Rainfall seasonality and an ecohydrological feedback offset the potential impact of climate warming on evapotranspiration and groundwater recharge

The potential impact of projected climate warming on the terrestrial hydrologic cycle is uncertain. This problem has evaded experimentalists due to the overwhelming challenge of measuring the entire water budget and introducing experimental warming treatments in open environmental systems. We present new data from a mesocosm experiment that examined the combined responses of evapotranspiration (ET), soil moisture, and potential groundwater recharge (R; lysimeter drainage) to a 3.5 degrees C temperature increase in a grassland ecosystem experiencing a Mediterranean climate. The temperature increase was applied both symmetrically throughout the day, and asymmetrically such that daily minimum temperature was 5 degrees C greater than ambient and daily maximum temperature was 2 degrees C greater than ambient. Our results span 3 water years and show that symmetric and asymmetric warming-enhanced ET during the spring. However, this increase in ET reduced soil moisture more rapidly, resulting in less ET during the summer than occurred under ambient temperature, and no difference in total ET during the combined spring and summer (March to October). Groundwater recharge was reduced during late-spring storms relative to the ambient temperature treatment, but these reductions were less than 4% of total annual R, and were offset by slightly greater R in the fall under both warming treatments. The results highlight the potential for local interactions between temperature, vegetation, and soils to moderate the hydrological response to climate warming, particularly in environments where precipitation is seasonal and out of phase with the vegetation growing season. Key Points Examined the vadose zone water balance response to imposed climate warming treatments Warming increased spring ET, but decreased summer ET, with no significant difference annually Warming had no effect on mean annual groundwater recharge in this climate